Isolated Variac Project Update

[SCSKITS]

You did not mention your output current requirements. If low enough, inrush protection may not be needed. Output voltage and current meters (or the digital meter equivalent) should be adequate. I can calculate power if needed. The switching scheme is overkill. The fuse (or maybe a circuit breaker) should be adequate. There are a number of small low amp circuit breakers available, but they may not be as fast as a fuse.

Attached a picture of the old Heathkit IP-5220 schematic. I have used this unit with no issues for years. The schematic shows two output connectors, mine is different with one connector attached to the variac output (not isolated) and the other as shown on the isolated output. And I have blown the 1A fuse on many occasions. I usually do not use the 3A range.

[dazz1]

Hi
I have been developing the circuit.  This has led me to AN4993 How to implement an SCR or a Triac in hybrid relay applications.
So I am moving to a hybrid Triac / electro-mechanical relay solution. 

The Application Note offers an SCR based solution for low RMS loads.    I seems to be that a Variac with no connected load is likely to struggle to turn off a triac.

Does anyone have anything to add to AN4993?

[djerickson]

I built one of these a few years back. Since I'll always use mine with 120VAC input (USA!), I put the isolation transformer after the 120V variac, That way I could also use it for a 0- 260V out supply by switching the transformer secondary. And either 120V or 240V, you get the variac and transformer full power.
Dave

[dazz1]

Hi
I ran an experiment today to measure the no-load magnetizing current for my Variac.
I had to rig a soft start circuit to limit the fuse blowing inrush current.
I used a halogen 240VAC lamp to limit the current, then manually switched it out of circuit.
My multimeter measures 10amps max.  I added a 1 ohm resistor as a current sensor.

The no-load magnetizing current is 30mA, 7W.  With this info I need to figure out if there is enough no-load current to use triacs for the soft starter.